Conventional mechanically driven valve trains of internal combustion engines operate the intake and exhaust valves based on the position and profile of lobes on a camshaft. The engine crankshaft is connected to the pistons by connecting rods and to the camshaft by a belt or chain. Therefore, the intake and exhaust valve opening and closing events are based on the crankshaft position. This relationship between the crankshaft position, piston position, and valve opening and closing events determines the stroke of a given cylinder, e.g., for a four stroke engine the intake, compression, power, and exhaust strokes. As a result, engine starting and the first cylinder to fire are affected in part by the camshaft/crankshaft timing relationship and the engine stopping position.
On the other hand, electromechanically driven valve-trains do not have the physical constraints that tie the camshaft and crankshaft together, i.e., there may not be belts or chains linking the camshaft and crankshaft at least for some valves. Furthermore, full or partial electromechanical valve-trains may not require a camshaft. Consequently, the physical constraints linking the camshaft and crankshafts are broken. As a result, additional flexibility to control valve timing is possible when electromechanical valves are used in an internal combustion engine.
One method to control electromechanical valve operation during an engine start is described in U.S. Pat. No. 5,765,514. This method provides for closing the intake and exhaust valves then allows the starter to crank the engine. If a signal pulse representing crankshaft rotation through 720 degrees has been generated, an injection sequence for each cylinder and a crankshaft position sequence are set. The injection sequence for the cylinders is initialized when a first crankshaft pulse is generated after generation of a first signal pulse representing crankshaft rotation through 720 degrees. The injection sequence and crankshaft position sequence correspond to the position of each cylinder, whereby the opening/closing timing of each intake valve and exhaust valve can be controlled. The cylinders are set to the exhaust stroke, intake stroke, compression stroke, and power stroke, respectively.
The above-mentioned method has several disadvantages. First, the method traps a volume of air in the cylinder, and as the engine rotates, this volume of trapped air is compressed and expanded until the respective cylinder stroke is set. Mechanical work is required to compress the trapped volume, which in turn increases fuel and power consumption, and can affect engine vibration.
Secondly, the method may produce inconsistent engine crank times. Depending on engine stopping location, up to 720 degrees may be necessary to determine engine position. On the other hand, engine position may be identified relatively quickly depending on where the engine stopped relative to the 720-degree sensor.
The inventors herein have recognized these disadvantages of the before-mentioned approach. The inventors also have considered these limitations and determined that the before-mentioned approach simply focuses on operating cylinder valves based on conventional four-stroke engine operation and crankshaft position. With the exception of closing valves before cranking, the method operates intake and exhaust valves during a start similar to a conventional mechanically driven valve system. The method does not recognize that operation of intake and exhaust valves does not have to assume four-stroke timing during an engine start. Therefore, the approach overlooks opportunities to reduce engine emissions, vibration, and noise.
One embodiment of the present disclosure includes a method for starting an internal combustion engine with at least a valve that may be deactivated, the method comprising: processing a signal indicative of a request to start said engine; closing at least an exhaust valve of at least a cylinder in said engine in response to said signal; and maintaining said exhaust valve in said closed position until after a combustion event in said cylinder. This method can be used to reduce the above-mentioned limitations of the prior art approaches.
E.g., by closing an exhaust valve(s) of an engine before the engine begins to rotate and then holding the respective valve(s) closed until combustion has occurred in the cylinder, emissions may be reduced.
For example, for a port fueled engine with a conventional cam, expelled hydrocarbons can be reduced by delaying fuel delivery during cranking until combustion is desired. Specifically, if the exhaust valves are open when the cylinder cycles through an exhaust stroke during a start, residual hydrocarbons accumulated in the cylinder from cylinder wall oil film or fuel drawn into the cylinder during the last engine shut-down cycle may be expelled into the exhaust system increasing emitted hydrocarbons.
Hydrocarbons that are not combusted during cranking can be a significant portion of the overall starting emissions of a vehicle. As the number of cylinders in an engine increase, e.g., 14, V6, V8, the potential for expelling a higher mass weight of uncombusted hydrocarbons also increases.
The present disclosure may thus provide several advantages. For example, the method can reduce hydrocarbon emissions during a start by retaining residual hydrocarbons until combustion has occurred in the respective cylinder. Further, the method can reduce the amount of raw hydrocarbons that collect on a catalyst substrate that result from engine cranking. If raw hydrocarbons adsorb onto a catalyst, the amount of time before an efficient operating temperature is reached may increase due to the heat of vaporization of the fuel. Therefore, retaining uncombusted hydrocarbons in a cylinder until combusted can increase catalyst efficiency.
The above advantages and other advantages, objects and features of the present invention will be readily apparent from the following detailed description of the embodiments when taken alone or in connection with the accompanying drawings.